In GPRS (General Packet Radio Service), as described in 3GPP TS 23.060, the UE (User Equipment) can be in three different modes or states:
In GPRS IDLE state, the wireless terminal (mobile station, MS or User Equipment, UE) of a subscriber is not attached to GPRS mobility management (MM) of the network. The wireless terminal and Serving GPRS Support Node (SGSN) contexts hold no valid location or routing information associated with the subscriber.
In STANDBY state, the subscriber is attached to GPRS mobility management. The wireless terminal (UE) and SGSN have established GPRS mobility management (GMM) contexts.
In READY state, the SGSN MM context corresponds to the STANDBY MM context extended by location information associated with the subscriber on the cell level. The wireless terminal (UE) performs mobility management procedures to provide the network with the actual selected cell. GPRS cell selection and re-selection is generally done locally by the wireless terminal (UE), or may optionally be controlled by the network.
Regardless of whether a radio resource is allocated to the subscriber or not, the GMM context remains in the READY state even when there is no data being communicated. A timer (GMM READY timer T3314) supervises the READY state. The GMM READY timer is initiated or started when data is sent by the UE and the wireless terminal goes into the READY state. A GMM context moves from READY state to STANDBY state when the GMM READY timer expires. In order to move from READY state to IDLE state, the wireless terminal (UE) initiates the GPRS Detach procedure. More details can be found in 3GPP TS 23.060, sub-clause 6.1.1. The length (hence duration) of the GMM READY timer can only be changed by the SGSN.
For intersystem change between UTRAN and GERAN, the GMM READY timer T3314 can be kept running due to Cell Change Order (CCO) procedure completion, as described in 3GPP TS 44.060, sub-clause 8.4.1:
“In case of inter-RAT network controlled cell reselection, the mobile station shall regard the network controlled cell re-selection as successfully completed according to specifications of the target RAT, or when the GMM READY timer (see 3GPP TS 24.008) stops running during the execution of the procedure. The mobile station shall then stop timer T3174.”
It should be noted that 3GPP TS 23.060, sub-clause 6.13.1.1, which describes Iu mode to A/Gb mode Intra SGSN Change, only requires that the UE checks the PMM state in UTRAN to decide which procedure to trigger after intersystem change from UTRAN to GERAN, i.e. there is no checking of the Ready timer due to an intersystem change from UTRAN to GERAN beyond the case of the CCO procedure completion as highlighted above.
Evolved UTRAN (E-UTRAN) specifies an Idle mode Signalling Reduction (ISR) function which provides a mechanism to limit or reduce signalling in idle mode during any inter-RAT cell-reselection between E-UTRAN and UTRAN/GERAN. By keeping a context for a wireless terminal (User Equipment, UE) in both the MME and the SGSN, the UE does not need to perform NAS registration procedure (Routing Area Update, RAU) when moving between E-UTRAN and GERAN/UTRAN as long as the UE stays in the registered areas (Routing Area in GERAN/UTRAN, tracking Area List in E-UTRAN).
According to this mechanism the UE in idle mode, when ISR is activated, is registered with both the MME of the E-UTRAN and the SGSN of the UTRAN/GERAN (see 3GPP TS23.401, Annex J1). Both the SGSN and the MME have a control connection with the serving gateway (S-GW).
The UE receives and stores mobility management (MM) parameters provided to the UE by the SGSN (e.g. P-TMSI and RA) and provided to the UE by the MME (e.g. GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common to E-UTRAN and GERAN/UTRAN accesses.
Using these stored parameters and contexts, the UE when it is in idle state can reselect between E-UTRAN and GERAN/UTRAN radio access cells when the UE is within the registered routing areas (RAs) and tracking areas (TAs) without any need to perform any tracking area update (TAU) or routing area update (RAU) procedures by interacting with the network.